To achieve solar cells that convert energy with efficiencies up to 40 percent, quadruple junctions are being investigated. Fabricating such cells is relevant for instance for space applications, because the total array weight and thus the launching cost can be reduced by increasing the energy conversion efficiency. For terrestrial applications the use of such cells results in a decrease of the total amount of cost per Watt, if the sunlight is concentrated.
Currently, double and triple junction cells are being fabricated by several companies using monolithically stacked cells, where germanium is used as a substrate material as well as an active layer. Fabricating monolithically stacked four junction cells and optimizing the current matching for obtaining a high-energy conversion at the end of life (EOL) is quite complex, considering that the various junctions degrade at a different rate.
To lessen the need for current matching, it is interesting to look at the possibility of mechanical stacking of cells. A four terminal stack consisting of a top cell of two junctions in combination with a separate bottom cell is a good compromise between interconnection complexity and growth complexity. Germanium is a suitable material for realizing this bottom cell, because of its low energy band gap, low weight, and relatively low substrate cost.
A stand-alone germanium cell can be used as a bottom cell as a part of a high efficient multi-junction solar cell.
Furthermore a germanium cell is itself an interesting option for use as a receiver in a thermo photovoltaic (TPV) system, where it is used to convert specific radiation from a heat source. The use of germanium in a TPV system is especially interesting because of its relatively low substrate cost compared to other low band gap semiconductors like GaSb. In order to make the application of the germanium solar cell in a TPV system feasible, it will also be essential to keep the processing costs to a minimum.
The main problem of the current germanium cells is the need for good passivation of the front and backside. A good front side passivation is especially critical in germanium cells, because Ge has a quite high absorption coefficient, which causes the light to be absorbed close to the surface and thus makes the device extremely sensitive to recombination at the surface.
Surface passivation can be achieved by applying a certain material on the surface, which fills the dangling bonds and thereby reduces the amount of recombination centers at this surface. For example, materials like silicon oxide, silicon nitride or amorphous semiconductors can be used. These layers can be applied by techniques like chemical vapor deposition (CVD) or sputtering. Depending on the chosen method significant differences in material properties and passivation behavior can be obtained. Especially important with respect to passivation are the amount of hydrogen in the layer and the damage to the surface that is caused by the deposition technique.
European Patent Application No. EP-A-374244 is related to a method of fabricating a solar cell from a silicon substrate on which a passivation layer consisting of silicon nitride is applied, after which contacts are created by applying a silver containing paste onto the passivation layer and ‘firing through’ the contacts, i.e., subjecting the substrate to a diffusion step, so that silver particles penetrate the silicon nitride layer and make contact with the substrate. The process conditions and the materials chosen for this process are, however, unsuitable for a germanium substrate.
In P. N. Luke et al., ‘Germanium Orthogonal Strip detectors with Amorphous-Semiconductor contacts’, 1999 IEEE Nuclear Science Symposium Conference Record, Seattle Wash., 25-28 Oct. 1999, a contact layer of amorphous germanium with thickness of 110 nm is sputtered onto the surface of a germanium detector. The amorphous semiconductor layer functions as a blocking junction and acts as a passivation coating for the germanium surface. The provision of contacts as required for solar cells is not discussed.
The formation of contacts after the passivation of a germanium solar cell front side is not obvious. The properties of the germanium substrate and possibly passivation layer should not be altered significantly during this process, which limits process conditions as for instance processing temperatures (preferably kept below 300° C.).